The drug aphidicolin inhibits the synthesis of DNA from vacccinia virus and HeLa cells. The primary effect of the drug is through interaction with DNA polymerase. Tests at times up to six hours after infection indicate that viral DNA synthesis (5 pfu/cell) is reduced to about 40, 24, 15 and 12 percent at drug concentrations of 5, 10, 15, and 20 MuM compared with controls. The same comparison shows that cell DNA synthesis is reduced to 3, 2, 1 1/2, and 1 percent at these drug concentrations. At 1 MuM drug, cell synthesis is about 6% of control values. At 20 MuM, aphidicolin does not reduce the titer of infectious virus at 56 hours after infection, but does produce small plagues. At 80 MuM, the titer appears to be reduced by several logs. The drug is extremely reversible for both DNA synthesis and production of infectious virus. Physical maps have been derived for phage 42, a bacterial virus with a circularly permuted DNA.